Entropy change linked to the martensitic transformation in metamagnetic shape memory alloys

The magnetic contribution to the martensitic transformation entropy change plays a crucial role in the transition properties of meta-magnetic shape memory alloys. This contribution leads to a common dependence of the total entropy change at the structural transformation, Delta S-tr, on the normalized temperature difference delta(tr)(A) = (T-C(A) - T-tr)/T-C(A) (where T-C(A) is the Curie temperature of the austenitic phase and T-tr is the transformation temperature) whose main feature is a decrease in Delta S-tr with increasing delta(tr)(A). A phenomenological model based on a Bragg-Williams approximation has been proposed to describe the observed behaviour in terms of the evolution of the magnetic contribution to the total entropy change. The proposed approximation provides a good fitting between predicted and experimental entropy changes obtained for different Ni-Mn-based alloy systems. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.